Stockholders' Equity - Open Market Share Repurchases Activity (Detail) (USD $)	1 Months Ended								5 Months Ended	9 Months Ended	15 Months Ended
	Sep. 30, 2014	Aug. 31, 2014	Jul. 31, 2014	Jun. 30, 2014	May 31, 2014	Apr. 30, 2014	Mar. 31, 2014	Feb. 28, 2014	Dec. 31, 2013	Sep. 30, 2014	Sep. 30, 2014	Jan. 31, 2014
Accelerated Share Repurchases [Line Items]
Total Number of Shares Purchased	197,759	181,509	188,612	207,243	211,848	269,972	241,091	110,239	289,839		1,898,112
Average price of common stock purchased	$ 45.66	$ 47.15	$ 48.54	$ 47.44	$ 47.20	$ 44.47	$ 46.85	$ 43.42	$ 39.49	$ 45.37	$ 45.37
Total Number of Shares Purchased as Part of Publicly Announced Plans or Programs	197,759	181,509	188,612	207,243	211,848	269,972	241,091	110,239	289,839	1,608,273	1,898,112
Dollar Value of Shares that may yet be Purchased under the Plans or Programs	$ 413,891,828	$ 422,921,757	$ 431,480,660	$ 440,635,906	$ 450,467,820	$ 460,467,412	$ 472,472,783	$ 483,768,078	$ 488,554,427	$ 413,891,828	$ 413,891,828	$ 488,554,427